1. Technical Field
The present invention relates to a piezoelectric resonator element. In particular, the present invention relates to a mesa-shaped piezoelectric resonator element in which the thickness of a resonator section is thicker than that of a peripheral section.
2. Related Art
A traditionally manufactured AT cut quartz crystal resonator element forms an excitation electrode in the center of a piezoelectric substrate using a characteristic in which vibrational energy is concentrated in an area having a large mass. According to such a configuration, excitation in a desired vibration mode may be ensured without affecting vibration characteristics, even when a structure supporting a peripheral section of the piezoelectric substrate is employed. However, as the need for miniaturization of resonators and oscillators in which the piezoelectric resonator element is mounted increases, miniaturization of the piezoelectric resonator element serving as the core of the resonators and oscillators is demanded. When the area of the piezoelectric substrate decreases, the difference in mass within the substrate also decreases. Therefore, it is difficult to concentrate the vibrational energy in the resonator section. As a result, piezoelectric resonator elements having a mesa shape, a bevel shape, a convex shape, and the like have been developed. In the piezoelectric resonator elements, a resonator section having a thicker board thickness than the peripheral section is formed on the board surface of the piezoelectric substrate to confine the vibrational energy within the resonator section. For example, the piezoelectric resonator element is that described in JP-UM-A-6-52230.
JP-UM-A-6-52230 is an example of related art.
The mesa-shaped piezoelectric resonator element does not have a tapered structure or a curved structure, as do the bevel-shaped or convex-shaped piezoelectric resonator element, but rather has a simple step structure. Therefore, the mesa-shaped piezoelectric resonator element can be processed by etching and is suitable for mass production. However, in the mesa-shaped piezoelectric resonator element, the boundary between the resonator section and the peripheral section is clearly separated by a level difference. As a result, depending on the position of the level difference, an unnecessary mode is coupled with a thickness sliding mode that is a main mode. The unnecessary mode is, for example, a flexural mode. Therefore, the vibration characteristics may deteriorate. In addition, in the mesa-shaped piezoelectric resonator element, it is known that the crystal impedance (CI) value tends to decrease when the level difference between the resonator section and the peripheral section is increased. However, when etching depth is too deep, unnecessary vibrations may easily occur in the resonator section. The etching depth is the depth of the etching performed to form the level difference. In addition, when the etching time is increased to increase the etching depth, undesired may occur with degradation of the protective film. In other words, a phenomenon may occur in which areas that should not be etched are etched. In addition, as a result of the effect of the undesired etching, variations may occur in the shape of the manufactured piezoelectric resonator element.